As concern for the environment grow, biodegradable polymers are becoming of increasing commercial interest as a substitute for conventional and less degradable synthetic polymers such as polyolefins and polyurethanes. The use of biodegradable polymers in areas of packaging and textiles is therefore steadily growing. Consequently, much research is directed to the synthesis of these biodegradable materials. Among the various biodegradable polymers, polylactide (PLA) is one of the most commonly used and studied.
PLA is an aliphatic polyester based on lactic acid, the latter being obtained by fermentation of sugars and/or starch. PLA is therefore derived from renewable vegetables and is biodegradable by composting.
PLA is prepared from lactide by ring-opening polymerization. Lactide, which is a dimeric cyclic ester of lactic acid is in turn formed by dehydrating aqueous lactic acid to form a mixture of oligomers, which mixture is then depolymerized to form lactide. In order to prepare PLA of a high quality, it is very important to use lactide of a high purity. The lactide needs therefore to be purified to a high extent before ring-opening polymerization may take place to prepare PLA of a high quality.
A variety of lactide purification processes are known, that usually comprise one or more integrated distillation, condensation and melt crystallization steps, like in U.S. Pat. Nos. 5,521,278, 5,357,034, 5,214,159.
However, none of them suggest improving the production of meso-lactide while producing L-lactide in high yield from the crude lactide stream to be purified.
It is also well known that with the increasing demand in PLA polymers, it appears that a range of PLA polymers with various concentrations in enantiomers L- and D- is more and more interesting; but there is a need for easy means to obtain such type of polymers, and mainly to avoid the implementation of a second polymerization unit for D-Lactide or meso-lactide.
Presently, for the usual applications involving L-Lactide, it is requested to have such L-Lactide with a high optical purity; a usual process to get such a purity often needs a step to separate meso-lactide. One has to do with controlling the proportion of D-enantiomers in the lactide stream that is taken for the polymerization step.
It is also important to control the ratio of L- and D-enantiomers in the lactide stream. It is known that removing meso-lactide from the lactide stream has the effect of reducing the proportion of D-enantiomers, which leads to the production of a more crystalline grade of polylactide. In new applications it is more and more interesting to obtain copolymers having a low content of D-enantiomers, so broadening the range of grades of PLA which are needed to cover the different applications where PLA may be used; those grades of PLA may contain various amounts of D-enantiomers ranging from about 1% by weight to more than 10% by weight, depending of the final use desired.
In usual process to prepare PLA, mostly on the basis of the L-lactide enantiomer, these processes comprise a purification step of the crude lactide to selectively separate the L-lactide enantiomer from the crude lactide stream.
Concerning the meso-lactide, it is also separated from the crude lactide stream, but the separated stream will also comprise certain impurities such as water, lactic acid as well as dimers and trimers of lactic acid.
With the usual methods of separation, like distillation and/or melt crystallization, there occurs a concentration of the impurities in the meso-lactide stream. Although it is of common practice to blend impurities containing meso-lactide stream with the pure L-lactide stream in proportion not higher than about 15% by weight, this leads to production of less crystalline polylactide, and this is the reason why much or all the meso-lactide recovered in such processes, is simply discarded, and therefore, this drastically reduces the overall yield in lactide. Solutions have already been envisioned in the art, but they are not satisfactory.
It is therefore an object of the present invention to provide a process to recover a purified meso-lactide either alone or in the presence of L-lactide, from a crude lactide stream, said purified meso-lactide having a limited amount of acid impurities, the content of which is expressed in the form of mEq/kg and is not exceeding 50 mEq/kg, preferably less than 30 mEq/kg and most preferably less than 15 mEq/kg, in order to blend such meso-lactide with purified L-lactide stream to prepare suitable PLA polymers useful for the different applications where the presence of D-enantiomers are needed.
Another object of the invention is to provide such a process where the polymer obtained with the recovered meso-lactide does not present the drawbacks of the prior art.
It is a further object of the present invention to provide a process enabling to use recovered meso-lactide for blending with L-lactide.
It is still a further object of the present invention to increase the overall yield in lactides (polymerizable lactides/crude lactides fed) up to high values equal to or higher than 90% and most preferably higher than 95%. By lactides, it is meant the L-lactide, the D-lactide and the meso-lactide.
Finally it is an object of the present invention to provide an apparatus to operate the process of the invention.
At least one of the objects is fullfilled by the present invention.
Applicants have now found that meso-lactide from which certain impurities have been substantially removed like water, lactic acid, lactic acid oligomers, catalysts derivatives, light and heavy colour impurities as for instance sugars, nutrients, proteins and amino acids, is a source of D-enantiomers to prepare suitable PLA grades containing L- and D-enantiomers.
The process of the invention for the recovery and production of meso-lactide from a crude lactide containing stream comprises the steps of:                a. Subjecting a starting crude lactide stream (A) to a first distillation step to obtain:                    a top stream (B) mainly containing meso-lactide,            a bottom stream (C) and            a side stream (D) mainly containing L-lactide and meso-lactide;                        b. Recovering the side stream (D) and subjecting said stream to a melt crystallization step to obtain:                    a first purified stream (E) mainly containing L-lactide and            a drain stream (F) mainly containing meso-lactide and L-lactide;                        c. Recovering top stream (B) and drain stream (F);        d. Subjecting top stream (B) and drain stream (F) to a second distillation step to obtain a second purified stream (G) containing L-lactide and meso-lactide.        